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This article was written on 01 Jun 2008, and is filled under Vitamins.

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Vitamins – Vitamin D

Vitamin D

Sun in the sky

Getting vitamin D from the sun: Correctly applied sunscreen reduces our ability to absorb vitamin D by as much as 90 percent; sunscreen takes a few minutes to have this vitamin D-dampening effect, however, so if you put it on just before you go outside, you will be able to get enough sun exposure. In northern latitudes, however, exposure to the sun in winter will not form vitamin D (due to the angle of the sun’s rays), so a supplement is advisable.

Vitamin D helps ensure that the body absorbs and retains calcium and phosphorus, both critical for building bone. Laboratory studies also show that vitamin D keeps cancer cells from growing and dividing, and plays a critical role in controlling infections.

Some preliminary studies indicate that insufficient intake of vitamin D is associated with an increased risk of fractures, and that vitamin D supplementation may prevent them, especially when vitamin D is taken in conjunction with calcium. It may also help increase muscle strength, which in turn helps to prevent falls, a common problem that leads to substantial disability and death in older people. Other early studies suggest an association between low vitamin D intake and increased risks of breast, colon, and other cancers, as well as increased risk of multiple sclerosis. A recent randomized trial among postmenopausal women showed significant reductions in cancer incidence among those randomized to vitamin D.

A promising report in the Archives of Internal Medicine suggests that taking vitamin D supplements may even reduce overall mortality rates: A combined analysis of multiple studies found that taking modest levels of vitamin D supplements was associated with a statistically significant 7 percent reduction in mortality from any cause. The analysis looked at the findings from 18 randomized controlled trials that enrolled a total of nearly 60,000 study participants; most of the study participants took between 400 and 800 IU of vitamin D per day for an average of five years. Keep in mind that this analysis has several limitations, chief among them the fact that the studies it included were not designed to explore mortality in general, or explore specific causes of death. More research is needed before any broad claims can be made about vitamin D, chronic disease prevention, and mortality. (For more information on vitamin D and chronic disease prevention, see Ask the Expert—Vitamin D.)

How much do I need? Vitamin D

The Institute of Medicine’s current recommended intake of vitamin D is 5 micrograms (200 IU) up to age 50, 10 micrograms (400 IU) between the ages of 51 and 70, and 15 micrograms (600 IU) after age 70. Optimal intakes are much higher, though, with at least 25 to 50 micrograms (1,000 to 2,000 IU) recommended for those over age 2. If the multivitamin you take does not have 1,000 IU of vitamin D, you may want to consider adding a separate vitamin D supplement, especially if you do not spend much time in the sun. In extremely high doses—hundreds of thousands of IU or more—vitamin D is toxic and can even cause death; but in adults, taking up to 2,000 IU per day as a supplement is safe. In fact, some people may need 3,000 or 4,000 IU per day for adequate blood levels, particularly if they have darker skin, spend winters in the northern U.S., or have little exposure to direct sunlight. If you fall into these groups, ask your physician to order a blood test for vitamin D.

Food sources: Very few foods naturally contain vitamin D. Good sources include dairy products and breakfast cereals (both of which are fortified with vitamin D), and fatty fish such as salmon and tuna. For most people, the best way to get the recommended daily intake is by taking a supplement, but the level in most multivitamins (400 IU) is too low; encouragingly, some manufacturers have begun adding 800 or 1,000 IU of vitamin D to their standard multivitamin preparations.

Getting vitamin D from the sun: Correctly applied sunscreen reduces our ability to absorb vitamin D by as much as 90 percent; sunscreen takes a few minutes to have this vitamin D-dampening effect, however, so if you put it on just before you go outside, you will be able to get enough sun exposure. In northern latitudes, however, exposure to the sun in winter will not form vitamin D (due to the angle of the sun’s rays), so a supplement is advisable.

References

1. Holick MF. Vitamin D deficiency. N Engl J Med. 2007; 357:266–81.

2. Gordon CM, DePeter KC, Feldman HA, Grace E, Emans SJ. Prevalence of vitamin D deficiency among healthy adolescents. Arch Pediatr Adolesc Med. 2004; 158:531–7.

3. Lips P, Hosking D, Lippuner K, et al. The prevalence of vitamin D inadequacy amongst women with osteoporosis: an international epidemiological investigation. J Intern Med. 2006; 260:245–54.

4. Bischoff–Ferrari HA, Willett WC, Wong JB, Giovannucci E, Dietrich T, Dawson–Hughes B. Fracture prevention with vitamin D supplementation: a meta–analysis of randomized controlled trials. JAMA. 2005; 293:2257–64.

5. Boonen S, Lips P, Bouillon R, Bischoff–Ferrari HA, Vanderschueren D, Haentjens P. Need for additional calcium to reduce the risk of hip fracture with vitamin d supplementation: evidence from a comparative metaanalysis of randomized controlled trials. J Clin Endocrinol Metab. 2007; 92:1415–23.

6. Bischoff–Ferrari HA, Dawson–Hughes B, Willett WC, et al. Effect of Vitamin D on falls: a meta–analysis. JAMA. 2004; 291:1999–2006.

7. Broe KE, Chen TC, Weinberg J, Bischoff–Ferrari HA, Holick MF, Kiel DP. A higher dose of vitamin d reduces the risk of falls in nursing home residents: a randomized, multiple–dose study. J Am Geriatr Soc. 2007; 55:234–9.

8. Wu K, Feskanich D, Fuchs CS, Willett WC, Hollis BW, Giovannucci EL. A nested case control study of plasma 25–hydroxyvitamin D concentrations and risk of colorectal cancer. J Natl Cancer Inst. 2007; 99:1120–9.

9. Munger KL, Levin LI, Hollis BW, Howard NS, Ascherio A. Serum 25–hydroxyvitamin D levels and risk of multiple sclerosis. JAMA 2006; 296:2832–8.

10. Lappe JM, Travers–Gustafson D, Davies KM, Recker RR, Heaney RP. Vitamin D and calcium supplementation reduces cancer risk: results of a randomized trial. Am J Clin Nutr 2007; 85:1586–91.

11. Autier P, Gandini S. Vitamin D supplementation and total mortality: a meta–analysis of randomized controlled trials. Arch Intern Med. 2007; 167:1730–7.

12. Giovannucci E. Can vitamin D reduce total mortality? Arch Intern Med. 2007; 167:1709–10.

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